Optical waveguide moldings made from silicon nitride and a method of detecting optical wavelengths using same

ABSTRACT

A transparent molded article for optical applications made from silicon nitride and a coating thereon of oxidic silicon nitride formed by chemical reaction of the silicon nitride with oxygen. The moldings can be shaped, depending on the area of application, to give various articles, for example optical waveguides, prisms, sensor elements or lenses, and are used in all areas where high optical demands are made.

This application is a division of application Ser. No. 08/127,987, nowU.S. Pat. No. 5,558,746, filed Sep. 27, 1993 which is a continuation ofapplication Ser. No. 07/690,656 filed Apr. 24, 1991, now abandoned.

BACKGROUND OF THE INVENTION

The invention relates to optical materials made from silicon nitride, toa process for their preparation by pyrolysis of polymeric silazanes, andto their use.

The silicon nitride-containing ceramic optical material is obtained bypyrolysis of polysilazanes, which are previously processed to give thedesired moldings. A surface oxidic coating can be produced by thepyrolysis process.

The pyrolysis of polysilazanes to give silicon nitride/siliconcarbide-containing ceramic material has already been described in theliterature (Ceramic Bulletin, Vol. 62 (1983), 904-915). Polysilazanesare generally prepared using chlorosilanes as starting material, whichare reacted with ammonia, primary or secondary amines or with disilazane(U.S. Pat. Nos. 4,540,803, 4,543,344, 4,535,007 and 4,482,669).

A further method of preparing polysilazanes is to react aminosilaneswith excess ammonia or excess primary amine. The aminosilanes arethemselves prepared by reacting chlorosilanes with amines (FR-A 1 25 83423). For example, tetrachlorosilane SiCl₄ and methylamine give thetetrakis(methylamino)silane Si(NHCH₃)₄ :

    SiCl.sub.4 +8 CH.sub.3 NH.sub.2 =Si(NHCH.sub.3).sub.4 +4 CH.sub.3 NH.sub.3 Cl

The aminosilane is subsequently reacted with excess ammonia, with allthe methylamino groups being replaced by NH groups. Viscous to highlyviscous polysilazanes are produced, which can be pyrolyzed to givesilicon nitride-containing material in a ceramic yield of from 72 to 79%by weight. The disadvantage of this process is the use of large amountsof alkylamine, half of which reprecipitates as alkylamine hydrochloridein the preparation of the aminosilane. The polymers prepared from theaminosilane are viscous and therefore difficult to process; fiberproduction is not possible.

A large number of optical materials are known from the prior art. Thematerials used hitherto (for example glass, polysiloxanes, polymethylmethacrylates (PMMAs), etc.) all have the disadvantage that theirpossible uses are limited by the upper temperature limits. Thus,polymethyl methacrylates can only be employed up to a maximum of 80° to90° C. and glass up to a maximum of 800° C.

In addition, in fiber optics, a coating completely surrounding thefibers must be applied. The application of coatings of this type, forexample by immersing the fibers in a solution containing a polymer indissolved form or by special spinning processes, is expensive andcomplicated.

SUMMARY OF THE INVENTION

The object was therefore to find a simple process for the production ofoptical moldings which are distinguished by good temperature andchemical resistance and by good mechanical properties.

The present invention achieves this object.

The present invention provides a process for the preparation of opticalmaterials from silicon nitride by pyrolysis of polymeric silazanes, bypressing the pulverulent polymeric silazanes to give moldings before thepyrolysis or first dissolving the polymeric silazanes in an organicsolvent, drawing fibers from this solution and pyrolyzing these fibersafter evaporation of the solvent, or by preparing melts of polymericsilazanes, converting these melts into moldings by casting, injectionmolding or extrusion and subsequently pyrolyzing the moldings, whichcomprises producing an oxidic coating on the silicon nitride moldings inan oxygen-containing atmosphere during or after the pyrolysis. Thepyrolysis is preferably carried out in an atmosphere containing ammoniaor an ammonia/inert gas mixture at temperatures of from 800° to 1400° C.

The silicon nitride optical molding produced by this process and havinga surface coating of oxidic ceramic contains up to a maximum of 1% ofcarbon, up to a maximum of 2% of hydrogen and up to a maximum of 10% ofoxygen, is highly transparent and amorphous if the temperature duringpyrolysis does not exceed the crystallization point. By contrast, theamorphous silicon nitride contains some silicon nitride as a crystallinephase if the temperature during pyrolysis exceeds the crystallizationpoint. The optical moldings produced according to the invention can beemployed at temperatures above 800° C. and are particularly suitable foruse as optical sensors or optical waveguides.

The optical moldings according to the invention are produced frompolysilazanes, which are converted into silicon nitride by pyrolysis.

Suitable polymeric silazanes are all silazanes which have ceramic yieldssignificantly above 30% by weight on pyrolysis in an ammonia-containingatmosphere. Polymeric silazanes of this type are described, inter alia,in U.S. Pat. No. 4,720,532, U.S. Pat. No. 4,482,669, DE-A-3 733 727 andDE-A-3 737 921.

A process of this type which yields solid polysilazanes which aresoluble in common solvents and/or are fusible and can therefore be spunfrom a solution and/or melt is described in DE-A-B 3 737 921.

Examples of starting materials employed in this process for thepolymeric silazanes are dialkylaminodichlorosilanes, which can beobtained from organyl trichlorosilane and dialkylamines; the startingmaterials are reacted with at least 3.5 mol of ammonia in aproticsolvents at temperatures between -80° C. and +70° C.

In this reaction, an ammonia molecule first reacts with two SiClfunctions to give an NH bridge between the two silicon atoms: ##STR1##

Oligomeric units are formed by this reaction. Subsequently, some of thedialkylamino groups are displaced from the silicon atom, formingcrosslinked polymeric silazanes. At the same time, the terminaldialkylamino groups are replaced by NH bridges, producing additionalcrosslinking.

The polymeric silazanes formed dissolve in all common aprotic solventsand contain the following structural units: ##STR2## where the sameradicals are possible for R* as for R, but R and R* may be identical ordifferent (different if more than one aminochlorosilane is reacted withNH₃),

R=C₁ -C₄ -alkyl, vinyl or phenyl,

R'=C₁ -C₄ -alkyl, and

x and y are the molar fractions of the two structural units, where x+y=1and x=0.7-0.95.

This controllable ratio between x and y determines the degree ofcrosslinking of the polymeric silazane and thus the viscosity andprocessability to the ceramic.

Pyrolysis of silazanes of this type gives amorphous, glass-clear siliconnitride containing traces of carbon, hydrogen and oxygen. The carboncontent is a maximum of 1% by weight, while hydrogen may be present upto 2% by weight and oxygen up to as much as 10% by weight.

The pyrolysis is carried out in an atmosphere containing ammonia or anammonia/inert gas mixture. Examples of suitable inert gases are thenoble gases argon and helium, but also nitrogen, with NH₃ /noble gasmixtures being preferred. The pyrolysis temperatures are in the rangefrom 800° to 1400° C. Temperatures above 1200° C., in particular from1200° to 1400° C., cause the formation of partially amorphousmicrocrystalline ceramic materials containing α-silicon nitride as acrystalline phase. Below 1200° C., highly transparent, colorless, purelyamorphous silicon nitride moldings are obtained.

Depending on the pyrolysis temperature, the pyrolysis product in thisprocess comprises virtually purely amorphous (below 1200° C.) orpartially crystalline (above 1200° C.) silicon nitride.

An advantage of the polysilazanes employed is their ability to be shapedto give three-dimensional moldings even before pyrolysis. The shapingcan be carried out here, for example, by extrusion, slip casting, meltspinning or pressing of pulverulent polysilazanes or by other knownprocesses.

The polysilazane moldings obtained in this way are subjected topyrolysis, the moldings being exposed to an oxygen-containing atmosphereduring or after the pyrolysis, an oxidic coating thus being producedthereon. In this way, core/cladding moldings can be produced, which areparticularly Suitable for optical applications.

The oxygen treatment can be carried out at the same temperatures as thepyrolysis, preferably above 1200° C., but in particular at temperaturesin the range from 1300° C. to 1400° C. The thickness of the oxidiccoating can be adjusted in a simple manner through the duration of theoxygen treatment. In this way, it is thus possible, by a time-savingprocess which is simple to carry out, to produce optical articles whichhave excellent adhesion of the cladding to the core and a definedcladding diameter without the cladding being applied to the moldings ina separate process step.

The optical material prepared according to the invention can, aftersuitable shaping, be employed, in particular, as optical waveguides.

The requirement for the cladding to have a lower refractive index thanthe core of an optical waveguide is satisfied by the moldings producedaccording to the invention since the refractive index of the outeroxygen-containing ceramic coating is in the range from 1.44 to 1.55,while that of the silicon nitride phase is from 1.65 to 2.00 (at λ=546nm), depending on the oxygen content of the silicon nitride ceramic.

A further advantage of these moldings obtained in this way is their goodtemperature and chemical resistance and their good mechanicalproperties. Thus, the moldings produced in this way can be employedwithout hesitation at temperatures above 800° C., even resisttemperatures up to 1700° C. for some time and can even be used at above1800° C. under an elevated nitrogen pressure.

BRIEF DESCRIPTION OF THE DRAWING

The following detailed description, given by way of example but notintended to limit the invention solely to the specific embodimentsdescribed, may best be understood in conjunction with the accompanyingdrawing, in which:

FIG. 1 schematically shows a cross section of an optical molding inaccordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, the moldings 10 according to the invention madefrom silicon nitride 11 having an oxidic coating 12 can be shaped,depending on the area of application, to give various articles (forexample optical waveguides, prisms, sensor elements or lenses) and areused in all areas where high optical demands are made.

EXAMPLES Example 1

A polymeric hydridochlorosilazane prepared in accordance with DE-A-3 733727 was spun through a 300 μm nozzle to give filaments. These filamentswere pyrolyzed in an ammonia atmosphere containing 2 vpm of O₂ up to afinal temperature of 1300° C. The silicon nitride filaments obtained hada diameter of 100 μm and an element composition which contained 3.4% byweight of oxygen, 0.8% by weight of hydrogen and 0.1% by weight ofcarbon in addition to silicon and nitrogen.

The cross-sectional surfaces of the fibers were studied byenergy-dispersed X-ray analysis (measurement of local concentrations ofelements). This revealed that the oxygen concentration at the surface ofthe fibers, i.e. at the periphery of the cross-section surface, washigher than in the interior of the fibers.

The refractive indices were determined using a two-beam interferencemicroscope:

Refractive index of peripheral region n_(p) =1.443

Refractive index of internal region n_(i) =1.652

This gives, for the numerical aperture NA=√n² _(i) -n² _(p),

NA=0.804.

Measurement of the optical attenuation gave 2 dB/cm.

Example 2

A 5% strength by weight solution in toluene (pre-purified, ketyl dried)was prepared from a polymeric hydridochlorosilazane prepared inaccordance with DE-A-3 733 727. This solution was subsequently used toapply ultrathin coatings on silicon wafers by spin coating under aninert gas atmosphere and clean room conditions. The silicon hadpreviously been subjected to ceramic etching in order to produce an SiO₂coating on the silicon wafer. The toluene was then allowed to evaporateslowly, and the wafer was lastly pyrolyzed in an ammonia atmosphere(purity 99.999%) up to 1300° C.

The thickness of the silicon nitride coatings achieved was in the rangefrom 0.1 to 4 μm. The coating thicknesses indicated were determinedusing an α-stepper. Scanning electron microphotographs showed pore- andcrack-free silicon nitride coatings.

Example 3

A silicon nitride fiber having a diameter of 100 μm and a length of 6cm, produced by the process described in DE-A-3 733 727, was bonded to aglass fiber of the same diameter. To this end, the end faces of thefibers were aligned with one another by pushing the fibers into a glasscapillary 2 cm in length. It was possible to fix the fibers by brieflocal heating of the glass tube. Mechanical stabilization was providedby a small Al₂ O₃ tube, into which the bonded fibers were pushed in sucha manner that the free end of the silicon fiber jutted out by about 4cm. The other end of the tube was subsequently sealed using ahigh-temperature adhesive in order to fix the fiber. The glass fiber,with a length of about 6 meters, was connected to a single-channelsimultaneous spectrometer or optionally to a photodiode detector.

This system was used to determine the spectral distribution and theintensity variations of the emitted light in a glass-firing furnace. Tothis end, the sensor was passed through a drilled hole into the firingchamber and was positioned in the immediate vicinity of the flame front.The sensor could be used as often as desired without any change takingplace in the use properties.

We claim:
 1. A transparent molded article for optical applicationscomprising silicon nitride and a coating thereon of oxidic siliconnitride formed by chemical reaction of the silicon nitride with oxygen,wherein at λ=546 nm the silicon nitride has a refractive index fromabout 1.65 to about 2.00 and the coating thereon of oxidic siliconnitride has a refractive index from about 1.44 to about 1.55.
 2. Anarticle as claimed in claim 1 wherein the silicon nitride is amorphous.3. An article as in claim 1 wherein the silicon nitride contains, astrace impurities, up to 1 percent by weight carbon, up to 2 percent byweight hydrogen and up to 10 percent by weight oxygen.
 4. An article asin claim 3 wherein the silicon nitride is amorphous.
 5. An article as inclaim 1 wherein the silicon nitride includes a crystalline phase.
 6. Anarticle as in claim 1 which can withstand a temperature above 800° C. 7.An optical waveguide comprising silicon nitride and a coating thereon ofoxidic silicon nitride formed by chemical reaction of the siliconnitride with oxygen; wherein at λ=546 nm the silicon nitride has arefractive index from about 1.65 to about 2.00 and the coating thereonof oxidic ceramic has a refractive index from about 1.44 to about 1.55.8. A method for detecting a wavelength of light comprisingpassing saidlight through an optical material connected to a spectrometer or aphotodiode detector, wherein said material comprises silicon nitride anda coating thereon of oxidic silicon nitride formed by chemicallyreacting the silicon nitride with oxygen, wherein at λ=546 nm thesilicon nitride has a refractive index from about 1.65 to about 2.00 andthe coating thereon of oxidic silicon nitride has a refractive indexfrom about 1.44 to about 1.55, and detecting the wavelength of light bythe spectrometer or the photodiode detector.